Nowadays, highly energetic charged particles are used for a wide variety of purposes. While in the beginning highly energetic charged particles were only used in scientific experiments, they are meanwhile used in: industry and medicine. As an example, highly energetic charged particles are used for surface hardening or the implantation of impurities into semiconductors. For medical applications, the treatment of cancer by highly energetic charged particles plays an increasingly important role.
As charged particles, in principle all types of charged particles can be used. In particular, leptons (electrons, positrons), hadronic particles (protons, helium cores, heavy ions, mesons, anti-protons) have to be mentioned. If the energies to be obtained (i.e. the speed of the particles) are comparatively low, usually linear accelerators are used. However, if the energies to be obtained are higher, linear accelerators would become too long and hence too expensive to reach such high energy levels. Therefore, as an alternative, circular accelerators (so-called synchrotrons) are used for accelerating (and even for storing) charged particles to very high energy levels.
If circular accelerators are used, the problem of how to introduce (inject) and to extract the particles into and out of the circular accelerator arises.
For performing this task, special (particle) switches are used. Typically, a combination of a so-called kicker magnet and a septum magnet is used. The kicker magnet is an extremely fast switching electromagnet (switching time typically in the order of 0.1 microseconds) that is selectively “distorting” the path of the particle beam. If the kicker magnet is switched off (undistorted path), the particle beam flies straight forward and continues to circle within the circular accelerator. If the kicker magnet is switched on, however, the particle beam is kicked sideward (hence the name kicker magnet) by a couple of centimeters. To further separate the two possible particle tracks, a so called septum magnet is used (which shows an essentially static magnetic field). The septum magnet is designed in a way that a first cavity is provided, in which a strong magnetic field (in the order of around 1 Tesla) is present while in a second cavity no or only a very small magnetic field is present. Depending on the “kicking distance” of the kicker magnet, the two cavities are typically separated by only a couple of centimeters. It, is easily understandable, that this is a difficult task to accomplish.
The standard design for septum magnets is an electric coil with a C-shaped iron yoke, wherein the gap in the C-shaped iron yoke is forming the area, in which the magnetic field is applied to the charged particle beam.
Other possible designs for septum magnets are described in the U.S. Pat. No. 4,939,493 and in the scientific publications “The Truncated Double Cosine Theta Superconducting Septum Magnet” by F. Krienen, D. Loomba and W. Meng, Nuclear Instruments and Methods in Physics Research A 283 (1989), pages 5-12 and in “The Superconducting Inflector for the BNL g-2 Experiment” by A. Yamamoto et al, in Nuclear Instruments and Methods in Physics Research A 491 (2002), pages 23-40.